guessing the most efficient way will be to run an agarose gel, cut out bands of your interest and recover. direct digestion by exonuclease or purification based on affinity columns are always not 100% clean.

Dear Yunfu Lin, 

    Make an agarose gel (preferred 1%)  load your whole samples (either 25ul or 50ul) in the wells and run the gel.  All the primers (forms primer-dimer in PCR), non-specific bands will be separated accordingly. So that it is very easy to cut bands of your interest and go for Gel Extraction. 

Here you can use the Qiagen Gel Extraction Kit  which will give quite good DNA concentration.

Hope this helps you.

Thanks guys. I forgot mentioning that it is also critical to avoid losing any DNA (or the synthesized products, which are not PCR products instead are linearly synthesized products). Gel extraction is good idea but I think it is also definite that I will lose some DNA.

i recently tried this product:

http://www.zymoresearch.com/dna/dna-clean-up/pcr-dna-clean-up-concentration/dna-clean-concentrator-5

I had a gel clean up product that was not concentrated enough for sequencing reaction and I could not make more. So I ran it through this column to concentrate the

@Yunfu,

Yes, it will lose some DNA, but to compensate the lost ones, you can do two PCR reactions (same stuff, or even 3) and isolate them altogether with gel-extraction kit.

For the synthesized product, can they be purified by company when you order it?

i agree with solution that the run of your PCR in Gel is the best way to remove your primers form your PCR products and using Qiagen purified kit always the best 

Make an agarose gel, load your whole product PCR  ( 50ul) in the wells and run the gel and go for Gel Extraction kit. You can use a lot of product PCR in the wells. 

For gel extraction, we found that the GENECLEAN® II kit (MP Biochemicals) protocol works really well and recovers a lot. Most of the reagent can be made and you just need to purchase the GLASSMILK.

http://kirschner.med.harvard.edu/files/protocols/MPBIO_geneclean2.pdf

I started off with Qiagen gel extraction kits but ended up choosing this due to ease of use and efficiency of recovery.

Yunfu,

Check this out

http://www.bio-rad.com/en-us/product/pcr-kleen-spin-purification-module

key word: spin column, gel fitration

Thanks. Many years ago, I used both Exo I and shrimp alkaline phosphatase to treat PCR to remove primers before I did manual sequencing. I found a "ExoSAP-it" kit that uses the same enzymes to digest primers. Has anybody tried to use this kit?